High in heat resistance and corrosion resistance as well as mechanical properties such as strength and rigidity, in spite of being light in weight, fiber reinforced composite materials, which consist mainly of a reinforcement fiber, such as carbon fiber and glass fiber, and a thermosetting resin, such as epoxy resin and phenolic resin, have been used conventionally in a wide variety of fields including aerospace, automobiles, railway vehicles, ships, civil engineering, construction, and sports goods. In applications requiring high performance, in particular, fiber reinforced composite materials incorporating continuous reinforcement fibers are used, and carbon fibers, which are generally high in specific strength and specific elastic modulus, are adopted as reinforcement fiber. As the matrix resin, on the other hand, thermosetting resins are generally used and in particular, epoxy resins are adopted frequently because of good adhesive with carbon fibers, high heat resistance, high elasticity modulus, and high chemical resistance, as well as very small cure shrinkage. In recent years, fiber reinforced composite materials have to meet more rigorous requirements as their applications increase. When applied to structural members such as for aerospace applications and vehicles, in particular, they are required to maintain necessary physical properties under high-temperature and/or high-humidity conditions. However, although advantageous from the viewpoint of being lightweight, common polymer based composite materials are not sufficiently high in heat resistance and limited to a small range of applications.
As curing agents for epoxy resins, aromatic amine compounds, acid anhydrides, and phenol novolac compounds have been used frequently in aerospace industries where high heat resistance is emphasized. These curing agents, however, tend to require a long heating period at a high curing temperature of about 180° C. in the molding step. When using an epoxy resin composition with a low reactivity, therefore, some problems will occur including a long molding time and high energy cost required for the molding step. Thus, good techniques have been called for to enable quick, low-temperature curing of epoxy resin compositions.
In this connection, Patent documents 1 and 2 propose the use of cationic-polymerization type curing accelerators such as boron trifluoride-amine complexes and sulfonium salts to shorten the curing period for epoxy resins.
Patent document 3 describes that the use of a microencapsulated imidazole compound as a curing accelerator serves to shorten the curing time while exhibiting high storage stability at 25° C.
In Patent document 4, the use of a microencapsulated phosphorous curing accelerator serves to produce a highly heat resistant cured product while maintaining high storage stability at 50° C.
Patent document 5 describes that the use of a microencapsulated cationic polymerization initiator mixed with an epoxy resin serves to shorten the curing time while maintaining high storage stability.